The invention relates to power detection circuits, and more particularly, to power detection circuits which compensate for temperature variations.
Power detection circuits are used in numerous applications where it is necessary to provide detection and/or measurement of the average power of a high frequency signal. Depending on the application, it may be necessary for the power detection circuit to operate over a wide range of temperatures. In such an application, it is essential for accuracy of measurement and efficient operation that the power detection circuit provide a consistent output for a given power input over the range of temperatures.
However, a common component used within power detection circuits, namely a diode, is sensitive to temperature changes, e.g., a junction voltage change of approximately 1 millivolt per degree Celsius. As such, there have been attempts in the art to compensate for this temperature change. U.S. Pat. No. 4,970,456 discloses a temperature compensated power detector. In the ""456 power detector circuit, a first power detection diode is provided on the positive input of an amplifier and a second temperature compensating diode is used on the inverting input of the amplifier. Paths to each of the diodes are provided with resistances which are arranged in an attempt to bias each of the diodes at approximately the same points on their operating characteristics and thus, allowing the temperature compensating diode to cancel the temperature sensitive component of the power detection diode at the input to the amplifier.
The power detection circuit of the ""456 patent is a reasonable approach to temperature compensation, however, the reliability and effectiveness of the circuit is very much, and undesirably so, dependent on how well the diodes are matched as well as how well the biasing circuitry is matched.
U.S. Pat. No. 5,371,473 describes a thermally stable automatic level control circuit for pulsed output amplifiers which utilizes a temperature compensated power detection scheme. In the ""473 patent a single diode is used for power detection. In operation, the power detection scheme exploits the discontinuous pulse transmission to extract the thermally borne voltage of the power detector between transmission bursts, hold that voltage, and then subtract that voltage from the power detection voltage during a transmission burst to provide a true voltage.
The power detection scheme of the ""473 patent is also a reasonable approach to temperature compensation, however, sample and hold circuitry is required in addition to the power detector circuitry which complicates the overall circuit and adds to the overall cost.
In view of the above, there is a need for a temperature compensated power detector which is not overly reliant on the matching of electrical characteristics of its biasing circuitry, and which does not require sample and hold circuitry.
The needs above are in large measure solved by a temperature compensated power detector of the present invention. The temperature compensated power detector generally comprises a detector circuit portion, which includes a detector diode, and a temperature compensation circuit portion, which includes a temperature compensation diode; the temperature compensation circuit portion is operably connected to the detector circuit portion. The detector diode and the temperature compensation diode are connected in DC series with each other and develop substantially identical voltage drops. The detector circuit portion operates to detect a voltage from a power input. However, the detected voltage is subject to alteration due to temperature variations. The temperature compensation circuit portion develops a voltage that is also subject to alteration due to temperature variations. The temperature altered voltage of the temperature compensation circuit portion is used to cancel out the temperature altered voltage of the detector circuit portion allowing the power detector to produce a true voltage output.
A method for producing a substantially temperature independent voltage output that is representative of a power input generally comprises the following steps: (1) receiving the power input; (2) detecting the envelope voltage of the power input with a detector circuit portion that includes a detector diode; (3) temperature compensating the detected voltage with a compensator circuit portion which includes a temperature compensation diode that is in DC series with the detector diode; and (4) outputting the temperature compensated detected voltage.